1. Field of the Invention
The present invention relates to a liquid supply apparatus, an image forming apparatus and a liquid supply method, and more particularly, to a liquid supply apparatus, an image forming apparatus and a liquid supply method in which ink is supplied to a recording head from a main tank via a sub tank.
2. Description of the Related Art
An inkjet recording apparatus has been commonly known which records an image on a recording medium by ejecting ink droplets from a plurality of ejection ports (nozzles) formed in a recording head, while moving the recording head and the recording medium relatively to each other.
A method of supplying ink to the recording head has been widely used in which ink is supplied to the recording head from a main tank, via a sub tank. In this liquid supply method, it is possible to reduce variation of internal pressure in the recording head, and hence to improve the ejection stability of the recording head.
For example, Japanese Patent Application Publication No. 11-348300 discloses a method in which a sensor which measures ink level in a sub tank is provided so as to monitor the ink level and the negative pressure in the recording head is maintained by keeping the ink level within a target range. In this method, it is possible to carry out an ink replenishment operation to the sub tank, even when the recording head is being driven.
Moreover, Japanese Patent Application Publication No. 2002-001978 discloses a method in which the pressure in the sub tank is measured, and liquid is replenished from the main tank on the basis of the measured pressure.
However, with the improvement of image quality and the increase in recording speeds in recent years, the amount of ink consumed by the recording head has tended to increase, and the following problems have arisen with the methods described in Japanese Patent Application Publication Nos. 11-348300 and 2002-001978.
Firstly, in the method described in Japanese Patent Application Publication No. 11-348300, since the flow channel connecting the sub tank with the recording head is long, then the pressure loss is liable to become large depending on the increase in the flow rate from the sub tank to the recording head, and moreover, opening and closing of the valves in ink supply operation are liable to cause the pressure variation. Consequently, ejection characteristics of the recording head are liable to become instable.
Moreover, in the method described in Japanese Patent Application Publication No. 2002-001978, the recording operation (ejection operation) performed by the recording head is required to be halted before replenishing ink, and therefore, it is difficult to achieve higher-speed recording.